CHAPTER 2 REVIEW QUESTIONS
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Chapter Review Questions Chapter 2
1. Describe the electric charge, mass, and location of the three major subatomic particles in an atom.
The two charged particles are the electron, which has a negative charge and very little mass, and which orbits the atomic nucleus, and the proton, which is positively charged, has ~2000 times the mass of the electron, and is found in the nucleus. The other particle, the neutron, has no charge, has about the same mass as the proton, and is also found in the nucleus.
2. Which four kinds of atoms are most abundant in the body?
Hydrogen (H), oxygen (O), carbon (C), and nitrogen (N).
3. Describe the distinguishing characteristics of the three classes of essential chemical elements found in the body.
The four major elements listed above account for 99.3% of the total atoms in the body. There are seven essential mineral elements—calcium (Ca), chlorine (Cl), magnesium (Mg), phosphorus (P), potassium (K), sodium (Na), and sulfur (S). These are the most abundant substances dissolved in the body fluids. Thirteen other elements, called essential trace elements, are present in extremely small quantities but are nevertheless essential for normal body functions.
4. How many covalent bonds can be formed by atoms of carbon, nitrogen, oxygen, and hydrogen?
Carbon can form four, nitrogen three, oxygen two and hydrogen one.
5. What property of molecules allows them to change their three-dimensional shape?
Molecules are not rigid structures. Atoms can rotate around their covalent bonds to form different shapes.
6. Describe how an ion is formed.
Ions are formed when an atom either gains or loses one or more electrons. Atoms that gain electrons have a net negative charge and are called anions. Atoms that lose electrons have a net positive charge and are called cations.
7. Draw the structures of an ionized carboxyl group and an ionized amino group.
Carboxyl: O
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R — C — O –
Amino: H
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R — N+ — H
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H
8. Define a free radical.
A free radical contains an unpaired single electron in its outermost orbital, or a molecule containing such an atom.
9. Describe the polar characteristics of a water molecule.
Water has the structure: (+) (-) (+)
H — O — H
The bonds between oxygen and each of the two hydrogen atoms are polar, meaning that oxygen, with eight times as many protons as hydrogen, draws the shared electrons closer to its nucleus. Water molecules interact with each other through hydrogen bonds.
10. What determines a molecule’s solubility or lack of solubility in water?
For a molecule to dissolve in water (i.e., be hydrophilic), it must be electrically attracted to water molecules. In other words, it must have a sufficient number of polar bonds and/or ionized groups. Nonpolar molecules do not dissolve in water, and are said to be hydrophobic.
11. Describe the organization of amphipathic molecules in water.
Amphipathic molecules have two parts or domains: a polar region at one end and a nonpolar region at the other end. In water, such molecules form clusters so that their hydrophilic “heads” are oriented on the outside of the cluster and their hydrophobic “tails” are oriented toward the inside, away from the water molecules.
12. What is the molar concentration of 80 g of glucose dissolved in sufficient water to make
2L of solution?
Glucose has a molecular weight of 180. Eighty g of glucose is 180/80 = 0.44 mol/2 L. This is equivalent to 0.22 mol/L.
13. What distinguishes a weak acid from a strong acid?
Molecules that release hydrogen ions in solution are called acids. A strong acid releases 100% of its hydrogen ions, while a weak acid does not ionize completely.
14. What effect does increasing the pH of a solution have upon the ionization of a carboxyl group? An amino group?
Increasing the pH of a solution decreases the concentration of free hydrogen ion in that solution, and thus will favor increasing the ionization of weak acids, such as a carboxyl group. It will decrease the ionization of weak bases, such as an amino group.
15. Name the four classes of organic molecules in the body.
Carbohydrates, lipids, proteins, and nucleic acids.
16. Describe the three subclasses of carbohydrate molecules.
The basic unit of the carbohydrates is the monosaccharide, which has the chemical formula Cn(H2O)n, where “n” is any whole number. Two monosaccharides can join together to form a disaccharide. Polysaccharides are polymers of monosaccharides.
17. To which subclass of carbohydrates do each of the following molecules belong: glucose, sucrose, and glycogen?
Glucose is a monosaccharide; sucrose a disaccharide; and glycogen a polysaccharide.
18. What properties are characteristic of lipids?
Lipids are composed primarily of carbon and hydrogen atoms, which form nonpolar bonds. Thus, lipids are nonpolar and hydrophobic.
19. Describe the subclasses of lipids.
Triacylglycerols: Most of the body’s lipids are triacylglycerols, or “fat.” Each triacylglycerol is composed of three fatty acids linked to a three-carbon carbohydrate named glycerol. A fatty acid is a chain of carbon and hydrogen atoms with a hydroxyl group (an acid) at one end. If all of the bonds between the carbon and hydrogen atoms are single covalent bonds, the molecule is said to be a saturated fatty acid. Molecules with one or more double bonds is said to be unsaturated.
Phospholipids: A phospholipid is similar to a triacylglycerol except that in a phospholipid two fatty acid chains are linked to glycerol, with the sugar’s third hydroxyl group attached to a phosphate group. Often a polar or ionized nitrogen-containing molecule is attached to the phosphate. Phospholipids, therefore, have a polar region as well as nonpolar ends, and are amphipathic.
Steroids: These are composed of four interconnected rings of carbon atoms bound to hydrogen atoms and each other, with a variety of chemical groups attached to different places in the rings.
20. Describe the linkages between amino acids that form polypeptide chains.
Amino acids are linked together when the carboxyl group of one reacts with the amino group of another, forming a peptide bond and releasing a molecule of water. The carboxyl group of the second amino acid can react with the amino group of a third, and so on, forming a polymer called a polypeptide.
21. What is the difference between a peptide and a protein?
A peptide is a polypeptide with 50 or fewer amino acids; a protein is a polypeptide with more than 50 amino acids.
22. What two factors determine the primary structure of a polypeptide chain?
The primary structure of a polypeptide is its amino acid sequence, i.e., the number of amino acids and the specific type of amino acid at each position on the chain.
23. Describe the types of interactions that determine the conformation of a polypeptide chain.
The conformation of a polypeptide is its three-dimensional shape. It is determined by (1) hydrogen bonds between portions of the chain or with surrounding water molecules; (2) ionic bonds between polar and ionized regions along the chain; (3) van der Waals forces between hydrophobic regions of the chain and (4) covalent bonds, called disulfide bonds, between the side chains of the amino acid cysteine (not all polypeptides have disulfide bonds).
Hydrogen bonds between the hydrogen linked to the nitrogen in one peptide bond and the oxygen in another occur at regular intervals along the chain and coil it into a helical shape (alpha helix). Hydrogen bonds between peptide bonds running parallel to each other can force a straight structure called a beta sheet.
Some proteins, called multimeric proteins, consist of more than one polypeptide chain.
24. Describe the structure of DNA and RNA.
DNA and RNA are nucleic acids, deoxyribonucleic acid and ribonucleic acid, respectively. The subunits of both nucleic acids are called nucleotides, and consist of a phosphate group, a sugar (deoxyribose in DNA, ribose in RNA), and one of five possible carbon-nitrogen rings called purine or pyrimidine bases. Nucleotides are linked together by covalent bonds between the sugar and phosphate groups of adjacent subunits.
The three-dimensional structure of DNA is a double helix, with the two strands held together by hydrogen bonds between a purine base on one chain and a pyrimidine base on the other. RNA consists of a single chain of nucleotides.
25. Describe the characteristics of base pairings between nucleotide bases.
The bases in DNA are the purines adenine and guanine and the pyrimidines thymine and cytosine. Adenine binds only to thymine and guanine only to cytosine.
RNA can form base pairs with DNA, as above, except that RNA has the pyrimidine uracil instead of thymine. Uracil forms hydrogen bonds with adenine.
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